1. Field of the Invention
The present invention relates to an image forming apparatus for developing an electrostatic image formed on an image bearing member by use of a developer and recording the developed image on a transfer material, for example, a copying machine, a page printer, a facsimile, or the like.
2. Related Background Art
Up to now, an image forming apparatus adopting an electrophotographic process, such as a copying machine, a printer, or a facsimile, includes an electrophotographic photosensitive member (photosensitive member) that serves as an image bearing member and generally has a rotating drum shape, a charging apparatus (charging step) for uniformly charging the photosensitive member to a predetermined polarity/potential, an exposure apparatus (exposure step) serving as information writing means for forming an electrostatic latent image on the charged photosensitive member, a developing apparatus (developing step) for visualizing the electrostatic latent image formed on the photosensitive member as a developer image (toner image) by use of toner serving as a developer, a transfer apparatus (transfer step) for transferring the toner image from a surface of the photosensitive member onto a transfer material such as a paper, a cleaning apparatus (cleaning step) for removing the developer remaining to a certain degree on the surface of the photosensitive member that has undergone the transfer step (residual toner or transfer residual toner) to clean the surface of the photosensitive member, a fixing apparatus (fixing step) for fixing the toner image on the transfer material. The photosensitive member is repeatedly subjected to the electrophotographic process (including charging, exposure, developing, transfer, and cleaning) to form images.
In general, provided inside the cleaning apparatus is a waste toner collecting container for receiving the transfer residual toner removed from the surface of the photosensitive member by the cleaning apparatus. Therefore, in order to obtain the image forming apparatus with a long life, it is necessary to increase a size of the waste toner collecting container, which is a disadvantage in reducing the size of the image forming apparatus.
In view of the above, an image forming apparatus of a cleanerless system has been proposed which is structured such that, without using the cleaning apparatus having the waste toner collecting container, the transfer residual toner on the photosensitive member that has undergone the transfer step can be removed and collected from the photosensitive member to be reused in the developing apparatus by means of cleaning simultaneous with developing for simultaneously performing a developing operation and a cleaning operation.
In the cleanerless system, when the transfer residual toner on the photosensitive member that has undergone the transfer step is reused in the subsequent developing steps, that is, when developing another electrostatic latent image after the photosensitive member is again charged and exposed to form the electrostatic latent image, the transfer residual toner existing in a portion (unexposed portion or non-image portion) on the photosensitive member which is not to be developed can be collected in the developing apparatus by application of a fog removing bias (a fog removing potential difference Vback which is a potential difference between a DC voltage applied to the developing apparatus and a surface potential of the photosensitive member).
According to the above method, the transfer residual toner is collected in the developing apparatus and reused to develop electrostatic latent images in the subsequent developing steps. Therefore, the residual toner can be eliminated, and inconvenience of maintenance can also be reduced. In addition, the image forming apparatus of the cleanerless system is advantageous in reducing its size.
In recent years, particularly in an image forming apparatus capable of forming an image in color (chromatic color), there has been widespread demand for an ability to output an image at high speed and a compatibility with various kinds of transfer papers (media). To meet the demand, an image forming apparatus of a four-drum system (an in-line system) and of an intermediate transferring member system has been devised, which includes a plurality of image forming sections each having a photosensitive member serving as a first image bearing member, and temporarily multiple-transfers toner images formed on the respective photosensitive members onto an intermediate transferring member (a transfer member) serving as a second image bearing member in a continuous manner to obtain, for example, a full color print image.
However, the following problem occurs in an image forming apparatus that adopts the cleanerless system in terms of a long life and a small-sized apparatus, the intermediate transferring member system in terms of a compatibility with various kinds of media, and the in-line system in terms of an ability to output an image at high speed.
That is, in the structure adopting the in-line system, the cleanerless system, and the intermediate transferring member system, there may occur a problem of color mixture for toner in the case of forming a color image, which will be described in detail below.
In a first image forming section, a yellow image is formed on a first photosensitive member, and in a second image forming section, a magenta image is formed on a second photosensitive member. Then, a yellow toner image and a magenta toner image are sequentially transferred onto the intermediate transferring member as the transfer member by respective transfer means opposed to the first photosensitive member and the second photosensitive member.
In this case, while the yellow toner image transferred from the first image forming section onto the intermediate transferring member is passing a position of the transfer means of the second image forming section, part of yellow toner may be transferred to the second photosensitive member, which is so-called retransfer.
This retransfer phenomenon depends on conditions such as surface properties and a surface potential of the photosensitive member, a charge amount and a specific charge of the toner, and the structure and the transfer bias of the transfer means. If those conditions can be appropriately adjusted, the probability of occurrence of the retransfer may conceivably become almost 0%. However, it is practically difficult to eliminate the retransfer phenomenon completely.
If the retransfer phenomenon occurs, in the cleanerless system, the toner of the image forming section on an upstream side in a moving direction of the intermediate transferring member is mixed into a developing device of the image forming section on a downstream side. Then, if a contamination amount of the toner becomes larger, the color tint of the image is varied due to the color mixture.
In view of the above, in the structure adopting the in-line system, the cleanerless system, and the intermediate transferring member system, a transferring current value of the transfer means for transferring and fixing the toner on the photosensitive member onto the intermediate transferring member is set to a lower value than in a structure conventionally used in general which includes blade-shaped cleaning means (a cleaning blade) for scraping toner off while being in abutment with the photosensitive member (hereinafter, referred to as cleaning blade structure). Thus, the color mixture due to the retransfer is suppressed to the minimum.
FIG. 3 shows an example of a correlation of the transferring current value with a transferring efficiency and a retransferring efficiency (this shows a case where a longitudinal length of an operation part with respect to an intermediate transferring member of primary transfer means is 330 mm). As can be seen from FIG. 3, the retransferring efficiency maintains an almost constant value up to Ib indicated in a graph of FIG. 3 according to the increase in the transferring current value of the transfer means, and shows a tendency of increasing after Ib. On the other hand, the transferring efficiency shows a tendency of increasing up to Ia indicated in the graph of FIG. 3, and shows a tendency of decreasing after Ia.
Therefore, in the case of the cleaning blade structure, the transferring current value is normally set to a value between Ib and Ia of FIG. 3, for example, approximately 14 μA. In the cleaning blade structure, the retransferred toner is collected by the cleaning blade, so that the problem of color mixture does not occur. On the other hand, in the cleanerless system, it is necessary to suppress the probability of occurrence of the retransfer to almost 0% as described above. Therefore, the transferring current value is normally set to a value that is equal to or lower than Ib of FIG. 3 and as high in transferring efficiency as possible, for example, approximately 8 μA. That is, as is apparent from FIG. 3, the conditions are set such that the transferring efficiency is slightly lower (worse).
Further, if the conventional toner is used to output images continuously, there is a case where the toner electrostatically coheres by a small amount. This is because in accordance with the continuous image output, a triboelectrification charge of a negative polarity (negative triboelectricity) amount of toner with a small particle size becomes larger while a small amount of toner with a large particle size tends to exert a triboelectrification charge of a positive polarity (positive triboelectricity), thereby causing the toner with a small particle size and the toner with a large particle size to electrostatically cohere.
In the image forming apparatus using the intermediate transferring member made of, for example, a resin, if the toner electrostatically cohere as described above, due to the hard intermediate transferring member made of a resin, a gap will develop between the photosensitive member and the intermediate transferring member. As a result, transferring properties are deteriorated.
In the case where the transferring properties are thus deteriorated, even if the transferring current value set for the cleaning blade structure exerts no adverse effect on the transferring properties, the transferring current value set for the cleanerless system may cause a problem of incomplete transfer.